package mo.leetcode_hot;

import java.util.Random;

public class L215 {

    private Random random = new Random();

    public int findKthLargest(int[] nums, int k) {
        return quickSelect(nums, 0, nums.length-1, nums.length-k);
    }

    private int quickSelect(int[] nums, int l, int r, int index) {
        int q = randomPartition(nums, l, r);
        if (q == index) {
            return nums[q];
        }
        return q < index ? quickSelect(nums, q+1, r, index) : quickSelect(nums, l, q-1, index);
    }

    private int randomPartition(int[] nums, int l, int r) {
        int i = random.nextInt(r-l+1) + l;
        swap(nums, i, r);
        return partition(nums, l, r);
    }

    private int partition(int[] nums, int l, int r) {
        int x = nums[r], i = l - 1;
        for (int j = l; j < r; j++) {
            if (nums[j] <= x) {
                swap(nums, ++i, j);
            }
        }
        swap(nums, i+1, r);
        return i+1;
    }


    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }

}
